We can debate whether those remaining undecideds, ranging from 3 to 8 percent in most of these polls, will break heavily for the challenger. In 2004, George W. Bush and John Kerry split the remaining undecideds roughly evenly. But the one scenario that political scientists deem virtually impossible is one where undecideds who have declined to support the incumbent all year suddenly break heavily in favor of him. For most of the remaining undecideds, the choice is between voting for the challenger and staying home.

There is some question about whether undecideds break evenly, or break largely against the incumbent, but it's worth noting there isn't much argument they break in favor of the incumbent. (By the way: If you want some analysis of that -- and why Nate Silver's model may have a very huge flawed assumption built into it -- read this guy's take on the Incumbent Rule.)

They make a point near the end: Obama has a lead in early voting, but that's based on their urging of high propensity voters to vote early. Those people would have voted anyway, early or on election day. Turning them out early doesn't really help the cause (unless... they vote twice, which is always a live possibility in Democratic machine politics).

Meanwhile, Romney is pursuing a strategy of pushing low- to medium- propensity voters to vote early, trusting that high propensity voters will turn out on their own. Such folks might be non-voters turned into voters.